Investigators are trying to understand the precise nature of the initial satcom log-on from Malaysia Airlines flight MH370, the first of seven exchanges which mark the only trace of the missing aircraft following its disappearance from surveillance radar.

The log-on request was initiated by the aircraft at 18:25UTC, just over an hour after the cessation of transponder and cockpit communications.

Australian Transport Safety Bureau investigators state, in a detailed analysis to help derive MH370’s subsequent track, that such a log-on was “not common” during cruise flight.

There are only a few reasons for a log-on, it says, among them a power interruption to the satellite data unit, loss of the link due to aircraft attitude, or a loss of critical systems.

Power interruption is considered a probable reason behind a similar aircraft-initiated log-on at 00:19 – the last of the seven satcom exchanges – a time when MH370’s fuel would have been nearing exhaustion.

The investigation states that flame-out of either engine would cut power to its corresponding AC electrical bus and that, once both engines stopped, power to the satellite data unit would have been temporarily interrupted.

Power would have been restored once the ram-air turbine deployed and the auxiliary power unit came on-line. The APU would have taken around 1min to start and the satellite data unit would have taken another 2min 40s to achieve the log-on at 00:19.

Interruption of the electrical power supply is “considered to be the most likely reason” for a satcom log-on request, says the inquiry.

But while fuel exhaustion and power loss could have generated the last log-on, the inquiry has yet to explain the first.

Power loss from either AC electrical bus would have automatically transferred power generation to its counterpart. But the inquiry indicates that this automatic switching would be “brief” and that the satellite data unit would have coped.

The inquiry says the initial log-on was “likely” to have resulted from a power interruption. But an interruption sufficient to generate a log-on request, it says, would have required either the loss of both electrical busses or a disabling of the automatic switching.

“As this power interruption was not due to engine flame-outs, it is possible that it was due to manual switching of the electrical system,” it states.

The inquiry offers no further analysis of this scenario beyond stating that the electrical system might have been operating in a different configuration from normal when the first engine flame-out occurred.

Around 90s after the first log-on request, the satcom log recorded communications from the in-flight entertainment system. A similar message would have been expected after the last log-on request but none was received – possibly indicating a complete loss of electrical power.

Five other satcom exchanges which occurred between the first and last log-on requests were all initiated by a ground station, and comprised routine interrogation messages to check that the connection to the aircraft was still operational.

Source: Cirium Dashboard